An actuator with a piston in piezoelectric materials has already been described in document FR 2 800 028.
As illustrated in FIG. 1, such an actuator includes a cylinder or a sliding sleeve 1 and a piston 2 designed to slide axially in the said cylinder 1.
The cylinder 1 is composed of several cylinders nested coaxially within each other, namely an outer cylinder 5, an inner cylinder 7 and an intermediate cylinder 6 which lies between the inner cylinder 7 and the outer cylinder 5, with the piston 2 sliding in the inner cylinder 7.
Cylinder 5 pre-stresses cylinder 7 and intermediate cylinder 6 by tightening onto them. This intermediate cylinder 6 includes radial slots 8, shown in FIG. 2, which extend from generating lines of the inner cylinder 7.
The piston 2 is composed of a multiplicity of piezoelectric ceramic sections 4. Each section is fitted with electrodes (not shown in FIGS. 1 and 2) which are used to control them independently of each other. Each section can be in multi-layer ceramic or in solid ceramic.
These electrodes are used to control the said sections, either to expand them transversally so that they are locked by friction onto cylinder 7, or to lengthen them, with the sections being operated in an alternating sequence of locking and lengthening so as to either move the piston or, when it is blocked, to generate a force which can then be used as a braking force for example.
Layers forming friction pairs are provided on the faces of the piston 2 and of cylinder 7, in order to ensure optimal friction.
The electrical elongation of a monocrystal piezoelectric element is the order of 1.4%. That of a multi-layer element is 0.1%. As an example, for an element measuring 25 mm, the resulting electrical elongation for a multi-layer element is about 9 μm. However, the manufacture of the multi-layer elements is much less costly and their operation allows the use of much lower voltages. With a control frequency up to an order of magnitude of 40 kHz, one can expect the same speeds of piston movement as with a hydraulic actuator.
However because of the very small elongation of the piezoelectric elements, control of the play between the sliding sleeve and the active piston in piezoelectric materials is vital. In fact, the locking force of the piston 2 in the cylinder 1 depends mainly on four factors:
the friction coefficient between the two parts 1 and 2 covered by the friction pairs 3;
the electrical expansion of the piezoelectric material of the piston 2;
the increase in the play between the cylinder 1 and the piston 2 due to the wear on these two parts;
the variations in the value of the play between the cylinder 1 and the piston 2 as a function of the system temperature.
This last factor shows that control of the play in the range of temperatures of the applications of such actuators, ranging from −40° C. (−60° C. for aircraft) to 200° C. or 300° C. or even more, represents a considerable challenge. This control determines the functional characteristics as well as the price of the actuator.
The material making up the active piston has a thermal expansion coefficient with an atypical value. Consequently, it must be possible, using conventional materials, to design a mechanical part which has a thermal expansion coefficient that is compatible with that of the active material.
The actuator of document FR 2 800 028 proposes to control the value of this play in accordance with the variation of temperature by exploiting different values of the expansion coefficients of the cylinders 5, 6 and 7.
To this end, the material(s) of cylinders 5 and 7 are chosen with expansion coefficients which are low but nevertheless algebraically greater than that of the piston 2. The material of the intermediate cylinder 6 is chosen with an expansion coefficient which is greater than the coefficients of parts 5 and 7.
When the temperature increases, the active piston contracts slightly. The play between the sleeve and the piston increases. In order to compensate for this increase, cylinder 6 expands radially, but it is partially impeded by cylinder 5 which expands less than it does. As a consequence, the constrained outward expansion is transferred to cylinder 7 which finds itself compressed radially.
The expansion slots 8 prevent the formation of orthoradial stresses, which would prevent any expansion of cylinder 5 toward the interior, and the compression of cylinder 7.
The internal expansion coefficient of cylinder 7 is adjusted by acting on the thickness of cylinder 5. Control of the play between the cylinder 1 and the active piston 2 in accordance with the temperature is therefore effected by choosing the relative thicknesses of the different cylinders 5, 6 and 7.
These actuators have drawbacks however. Their design, that is the determination of the different relative thicknesses, is difficult. In fact, cylinder 1 is composed of three cylinders, whose thicknesses constitute so many more parameters to be included in the design. Moreover, their manufacture is costly and complicated, since there is no solid lubricant at the interfaces between the cylinders to facilitate their relative sliding action due to thermal expansion, and their assembly in particular.
Document FR 2 819 468 proposes an actuator in which control of the play between the sleeve and the piston as a function of temperature is simplified. FR 2 819 468 proposes an actuator with a cylinder made from a crystalline material with a negative thermal expansion coefficient. Thus, the actuator of FR 2 819 468 includes an outer cylinder in which the sliding cylinder is pre-stressed, with either the outer cylinder or the sliding cylinder being in a material with a negative or approximately zero thermal expansion coefficient, with the other cylinder having a positive thermal expansion coefficient.
The crystalline materials presented in FR 2 819 468 for one of the cylinders have a thermal expansion coefficient that is negative isotropic or close to zero. These are chosen from amongst the following materials:
a. ZrW2O8, in α and/or β phase,
b. HfW2O8,
c. ZrV2O7,
d. YAIW3O12,
e. ZrP2-xVxO7, where 0≦x≦2, and
f. Sc2 (WO4)3.
These actuators also have drawbacks however, since they are limited in their area of use in respect of pressure and temperature.
In fact, in all the cases of use of the actuator, the hydrostatic tensions (or mechanical tensions according to the Von Mises criterion, created by the pre-stressing of the tubes) on materials with a negative expansion coefficient must not exceed 200 MPa (which is 2,000 bar). This pressure limitation for these materials means that the phase of the material is stable, and a phase transition is therefore avoided. The phase transition would cancel out the thermal expansion properties of the materials. Now when under mechanical stress in a brake or clutch device, a level of 200 MPa is reached with ease.
Moreover, the working temperatures are also limited, again so as to avoid a phase transition of the materials used. Table 1 (below) groups together the temperature and pressure characteristics of the different materials mentioned in FR 2 819 468.
A rise in temperature above the stability temperatures of the negative thermal expansion coefficient presented in table 1 cannot be neglected in a brake or clutch application.
The actuators of FR 2 819 468 therefore have areas of temperature and pressure that are of limited use, and sometimes incompatible with uses in the actuators of brake or clutch devices.
TABLE 1Coefficient ofThermal stabilityexpansion αarea of the expansionPhase[ppm/K)[° C.]α-ZrW2O8−8.8from −273 to +155β-ZrW2O8−4.9from +155 to +780(decomposition)YAIW3O12−5/−7<+1100ZrV2O70 from +150 to +700ZrP2−xVxO7  −6/−11.5 <+100Sc2 (WO4)3−2.2from +263 to +520
In addition, with a view to dimensioning of the parts (and in particular their radial thickness), a high absolute value of the negative thermal expansion coefficient is as important as thermal stability of the negative expansion. The materials of FR 2 819 468 sometimes do not have a negative thermal expansion coefficient with such a large absolute value.
Finally, the actuator of FR 2 819 468 has two cylinders, with one cylinder having a negative expansion coefficient in use and the other cylinder having a positive expansion coefficient. The fact that the material of one cylinder of document FR 2 819 468 has an isotropic negative expansion coefficient (meaning along all three axes of the crystalline material), and that the other cylinder is in a material that has an isotropic positive expansion coefficient means that there is a relative slippage of the cylinders which is quite significant along the longitudinal axis of the actuator. This relative slippage can damage the cylinders, despite the presence of a solid lubricant between the two cylinders.
In fact another element of FR 2 819 468 is the use of a solid lubricant in order to ensure axial slippage between the two cylinders comprising the actuator, with one cylinder having a negative expansion coefficient in use, and the other cylinder having a positive expansion coefficient.
The solid lubricants presented in FR 2 819 468 are chosen from                hexagonal boron nitride,        MoS2,        WSe2,        WS2,        graphite (carbon) interleaved with substances known to the professional engineer,        non-interleaved graphite,        tin sulphide (in its SnS, SnS2, and Sn3S4 forms), and Cerium fluoride (CeF3), or        any mixture of these materials.        
In a brake or clutch application, it turns out that in view of lengthy guarantees of avionic and/or automobile equipment, the action of water, humidity and steam on the solid lubricants becomes a new criterion to be included. Now the chalcogenides presented in FR 2 819 468, in particular the sulphides and the selenides, react with the water and lose their intrinsic lubricating character, which limits the use of the actuator.